Pressure transducers of the optical fiber type, commonly known as fiber optic pressure transducers, are becoming well known and, consequently, widely used, for and because of their ability to accurately sense pressure changes in specialized environments. In particular, the medical community is taking advantage of the sensitivity and quick response provided by these devices by using them to measure pressures within, for example, the blood system, the lungs, the urinary tract, and the cranial cavity.
Typically, these devices include a pressure responsive member having a first surface exposed to the pressure desired to be measured and a second surface exposed to ambient atmospheric pressure. The devices usually also include a housing that holds the pressure responsive member in a predisposed position relative to a housing aperture that allows the pressure responsive member to flex in the direction of least pressure in response to changing pressures. The housing also holds the distal end of an optical pathway in a predetermined position relative to the housing aperture and the pressure responsive member. The optical pathway includes a transmission path and a return path. Light is sent through the pathway on the transmission path from its proximal end to its distal end and then back through the pathway on the return path. Pressure deviations across the two surfaces of the pressure responsive member result in the movement of the pressure responsive member into and out of the path of the light travelling along the pathway, thereby modulating the return light signal by variably obstructing the light signal. This modulation of the light signal results in changes in the intensity of the light returning through the pathway. A control unit receives the returned light signal and then, after appropriate conditioning of the return signal, compares the intensity of the returning light signal with a correlation scheme that has been previously established between the measured intensity of the returned light signal and the pressure sensed by the pressure responsive member. This correlation scheme then provides an indication of the sensed pressure. Often, the pathway will include a reflector disposed at the distal end of the optical pathway in an optical coupling relation between the light transmission path and the light return path.
These fiber optic transducer devices can be made quite small and from biocompatible materials, thereby making their use with patients relatively comfortable as well as safe. Furthermore, they provide the attending clinician or physician with accurate, rapid responses indicative of pressure changes within the patient's system. These rapid responses are particularly advantageous where used with rapidly changing pressures.
Presently, most of these devices rely, as noted, upon a control unit that compares the sensed pressure, that is, the measured intensity of the returning light signal, with a previously established correlation between a particular light intensity and a measured pressure. A calibration cycle is run before first use so as to generate a correlation profile between later measured light intensities and known measured pressures. With these devices, the accuracy of the pressure measurement is completely dependent upon the accuracy of the predetermined correlation scheme established during the calibration sequence. This accuracy is in turn dependent upon the elasticity of the pressure responsive member. The deflection of the pressure responsive member is a function of the pressure differential experienced by the member divided by the member's elasticity constant. Thus, any change in the elasticity of the member during use will result in a change in the deflection and the modulation of the light beam, leading to pressure readings derived from the previously established correlation profile that deviate from the true pressure. Another problem with such correlation schemes is that they must cover a range of possible sensed pressures, and it is more likely that errors may occur in the correlation scheme at the pressure extremes. In addition, since the correlation scheme is established for ambient atmospheric pressure at the time the calibration sequence is run, it should be corrected as the ambient pressure changes or false readings of the sensed pressure may result. While not a great problem normally, situations do arise where rapid changes in atmospheric pressure do occur, especially when viewed within the time frame of a continuous monitoring of a patient.
Another concern of the foregoing known type of fiber optic pressure transducers is that construction of the transducer apparatus itself must be kept within critical parameters since the measured intensity of the returning light signal determines the sensed pressure. Thus, transducer construction becomes critical since the returning light signal must be linear and of a particular magnitude dependent upon the apparatus. Because a linear signal is needed, the foregoing known systems of calibration and control also require the use of linearization software on the returning light signal.
It would be desirable therefore to have a fiber optic pressure transducer that was less susceptible to errors creeping into the pressure measurement process due to inherent limitations in the pressure responsive member itself as well as the calibration system used to establish the correlation profile between the returning light signal intensity and the measured, that is, sensed, pressure.